The sound insulating properties of a wall, or door are reflected in the difference between the incident sound intensity level imposed on one face of the door and the transmitted sound intensity level emanating from the opposite face. The difference is called the "transmission loss". This loss is measured in decibels on a logarithmic scale with higher numbers indicating that the structure is better capable of insulating a space from outside noise.
One factor contributing to high transmission loss for a structure is the shear mass of the structure. The higher the mass, the greater the transmission loss. It is usually possible to build sufficient mass into the walls surrounding a quiet space to prevent the walls from transmitting unwanted noise. Doors to such spaces present special problems because it often is impractical or uneconomical to build sufficient mass into a door to provide the desired sound transmission loss. This is particularly true with respect to standard size doors commonly employed in conventional living and working spaces. Such doors are usually constructed of wood and have a thickness of only 13/4 inches.
It is not surprising, then, that some of the techniques of laminating layers of various materials, such as fiberboard, metal and glass wool, employed in producing sound insulating walls and panels have not been adapted successfully to acoustical doors. Examples of such panel constructions are disclosed in U.S. Pat. Nos. 2,116,270, granted May 3, 1938 to J. M. Le Grand for "BUILDING STRUCTURE" and 4,042,061, granted Aug. 16, 1977 to N. Murakami for "CELL-BOX-TYPE NOISE BARRIER HAVING LARGE MAGNITUDE OF TRANSMISSION LOSS AND NOISE INSULATING METHOD".
More recently it has been proposed to utilize a weighted plastic sheet suspended inside a door to help attenuate sound transmission. See U.S. Pat. No. 4,621,709, granted Nov. 11, 1986 to E. N. Naslund for "SOUND ATTENUATING PARTITIONS AND ACOUSTICAL DOORS". The manner in which the weighted plastic sheet is required to be loosely suspended inside the door substantially complicates the construction of a door embodying the invention of this patent.
There continues to be a need for a sound insulating door of conventional size and thickness which is economical to produce and which possesses a high sound transmission loss rating.